In the prior art, the electronic power steering apparatus for applying the steering assist force to the steering mechanism of the vehicle by driving the electronic motor in response to the steering torque that the driver applies to the steering wheel (handle) is employed. As the electronic motor of the electronic power steering apparatus, the brush motor is employed widely in the prior art. From the aspects of an improvement in reliability and durability, a reduction in inertia, etc., the brushless motor is also employed nowadays.
Commonly, when the brushless motor is driven at a high revolution and heavy load state, the voltage to be applied to the brushless motor becomes large. But actually the applied voltage to the brushless motor is subjected to restriction of the power supply voltage. Therefore, the control device for driving the brushless motor applies the control such that the applied voltage to the brushless motor does not exceed a predetermined voltage limit value (see Patent Literatures JP-A-2003-304697, WO2006/109809, JP-A-2008-29088, for example). As a result, when the revolution number N (per unit time) exceeds a certain value, an output torque of the brushless motor (referred to as a “motor torque” hereinafter) is decreased shown in FIG. 13(a), and a desired motor torque cannot be obtained.
As the method of suppressing a reduction in the motor torque in the high revolution range, a field-weakening control for supplying a negative d-axis current id to the brushless motor such that the magnetic flux of the revolving field as the rotor in the brushless motor in the d-axis direction is weakened is known in the prior art (see JP-A-2002-345281, JP-A-2006-248389, for example). When this field-weakening control is applied to the brushless motor, the motor torque Tm (N) is changed with respect to the revolution number N as indicated with a solid line FIG. 13(b), and a reduction in torque caused due to an increase in the revolution number N can be suppressed.